A prospective randomized controlled study of liver-directed and systemic adjuvant therapy in patients with high risk colon (excluding rectal) cancers is being carried out by the Gastrointestinal Tumor Study Group. Patients are randomized to no adjuvant therapy or to 2100 R of liver irradiation combined with 3 days of IV 5-FU. Ninety patients have been entered since the study was begun in August 1979. No conclusions regarding efficacy can be made at this early date, only 3 patients having relapsed on each arm. Myelosuppression has been more striking than expected and, in particular, thrombocytopenia has been occasionally striking. It is assumed that this toxicity reflects alteration of hepatic metabolism of 5 FU by liver irradiation. Since modification of the 5 FU dose last year, this toxicity has been reduced. Approximately 2-3 more years of accrual will be required.